ALL the women in the world were hogged by just a few men until around 10,000 years ago. That, at least, is the latest theory proposed to explain a mystery thrown up by genetic studies.
The history of the sexes is recorded in the Y chromosome, which is passed from father to son, and mitochondrial DNA, which is always inherited from the mother. Recent studies have revealed surprising differences in the patterns of diversity in mitochondrial DNA and in Y chromosomes. One explanation is that the female population boomed about 100,000 years ago – around the time that modern humans are thought to have spread out of Africa – whereas the male population expanded only recently, around 12,000 to 18,000 years ago.
But that would mean there were far more women than men for much of prehistory, which is simply impossible, says Guido Barbujani of the University of Ferrara in Italy. His team has now reanalysed data on 2000 Y chromosomes, looking at different types of mutations. Unlike previous studies, which looked only at European men, the analysis included 1000 samples from around the world. Yet the team still found similar differences in diversity compared with mitochondrial DNA.
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Barbujani and his colleague Isabelle Dupanloup propose that the apparent disparity between male and female populations could be explained if polygamy was once the rule rather than the exception, before the advent of agriculture led to a switch to monogamy around 10,000 years ago (Journal of Molecular Evolution, vol 57, p 85). They point out that if there were just as many men as women, but only a limited number of men begat most children, it would appear as if the male population was lower.
“The hypothesis is reasonable,” says Luigi Luca Cavalli-Sforza of Stanford University, famed for his work on human genetic diversity. In 1998, however, a team led by Cavalli-Sforza proposed that the different patterns of diversity in mitochondrial DNA and Y chromosomes could be explained by different patterns of migration (91av, 31 October 1998, p 11). Men tended to stay where they were born, the team suggested, while women often moved away when they married.
“There’s no reason why both polygyny and different migrational tendencies for the two sexes should be viewed as incompatible,” says Steve Jones, a geneticist at University College London. But like Cavalli-Sforza, he points out that polygamy seems to be less common in modern hunter gatherers than in settled peasant societies.
And Mark Seielstad, one of the authors of the 1998 paper, thinks agriculture allowed polygyny to become more common, not less, as it enabled some individuals to acquire more resources than others. He says his team did consider the possibility that the variation was due to polygyny, but concluded that it alone could not explain the differences. And he thinks the latest work is flawed, because the methods used did not take into account rare mutations. “The data analysed are woefully inadequate,” he argues.
Barbujani says there is no easy way to test the relative contributions of polygyny and migration patterns. But he hopes that computer simulations being developed will help narrow down the possibilities.